This invention relates to superconductor multi-chip modules (xe2x80x9cMCMxe2x80x9d) and, more particularly, to increasing the effectiveness of transmission of SFQ pulses between superconductor integrated circuits (xe2x80x9cchipsxe2x80x9d) within the module and enhancing bandwidth of the transition between the chip and the microstrip transmission line.
The most promising superconducting digital circuits communicate by transmitting Single Flux Quantum (xe2x80x9cSFQxe2x80x9d) pulses. The time integral of the voltage of a single flux quantum pulse is a physical constant, the flux quantum, approximately equal to 2.07 millivolt picoseconds or, in alternate terms, 2.07 milliamp picohenry. SFQ pulses are very fast and very small, having a time-integrated voltage equal to a flux quantum.
Modern superconductor digital circuits have been fabricated using integrated circuit (xe2x80x9cICxe2x80x9d) technology to form superconductor integrated circuits, a superconductor IC xe2x80x9cchipxe2x80x9d. Those IC chips are designed for operation at very high speeds (i.e.,. frequencies) of 20 Gpbs, 40 Gpbs, and up to 100 Gpbs. Multiple superconductor electronic devices are typically included on a single chip. Those devices are arranged in a circuit wherein signals, SFQ pulses, are propagated from one device on the chip to other devices on the chip to produce the function intended for the circuit. The on-chip medium through which those SFQ signals propagate has been the familiar Josephson Transmission Lines (xe2x80x9cJTLxe2x80x9d), which, as is known, is an electronically active line formed of Josephson Junctions, the active elements of that transmission line.
More recently, on-chip SFQ pulse propagation has been extended to superconducting microstrip having impedance of between one and ten ohms and of arbitrary length. That is, superconductor microstrip lines of appropriate characteristic impedance have been integrally formed on a superconductor IC chip. Being formed of superconductors, the microstrip is loss-less, and, containing no active electronic element, is passive in nature. Thus, the microstrip line is found to provide a less technologically complex transmission media for transmission of SFQ pulses between the various circuits on a chip than a Josephson transmission line. Because the microstrip line is passive in nature, the line requires no power source to operate. Finally, signal propagation on the microstrip line approaches the speed of light, which is much faster than propagation speeds on the JTL.
As semiconductor electronic systems became more complex, it was not always feasible to include all of the functional devices for the electronic system on a single chip. Instead, the electronic systems were produced using multiple chips with appropriate signal paths between the chips. Those chips were mounted to a common passive substrate and packaged together in a module, referred to as a Multi-Chip Module (xe2x80x9cMCMxe2x80x9d).
Similarly, the electronic circuits of superconductor digital systems are also increasing in functional complexity, and, following the lead with the prior semiconductor circuits, multiple superconductor chips were mounted on a common substrate and packaged together, defining a superconductor Multi-Chip Module. Passive transmission line, microstrip, was included to transmit a signal between individual chips in the module. Such is taught in Abelson, Elmadjian, Kerber and Smith, xe2x80x9cSuperconductive Multi-Chip Module Process for High Speed Digital Applicationsxe2x80x9d, IEEE Transactions on Applied Superconductivity, Vol. 7, No. 2, June 1997 pp 2627-2630
Further, a technology was developed and used to solder the semiconductor chips to the substrate of the MCM, referred to as xe2x80x9cflip-chipxe2x80x9d ball grid array, and/or mount the MCM to a circuit board, referred to as a ball grid array. An IC chip typically contains a large number of electrical interconnections that are dispersed over a flat side of the chip, forming an array of contacts for the electrical interconnections. Those electrical interconnections were difficult or impractical to solder individually. A preferred known technique for joining chips and making the electrical connections to wiring on a substrate is the flip-chip solder xe2x80x9cballxe2x80x9d or, as variously termed, solder xe2x80x9cbumpxe2x80x9d technique. In that technique solder bumps are fabricated at designated locations on the top of the chip (that correspond to locations of the solder pads on the substrate), the chip is inverted and placed on the substrate with the solder bumps aligned with corresponding solder pads on the substrate, and the solder is re-flowed by heating in an infra-red, convection, or vapor phase oven or on a hot plate to solder the chip in place on the substrate. The foregoing technique electrically and mechanically bonds the solder balls to the associated bonding pads on the circuit board, forming respective solder joints. The foregoing solder bump technique was also adopted for mounting of superconductor chips to a substrate. See Yokoyama, Akerling, Smith, and Wire, xe2x80x9cRobust Superconducting Die Attach Processxe2x80x9d, IEEE Transactions on Applied Superconductivity, Vol. 7, No. 2, June 1997 pp 2631-2634 and Maezawa, Yamamori and Shoji, xe2x80x9cDemonstration of Chip-to-Chip Propagation of Single Flux Quantum Pulsesxe2x80x9d, IEEE Transactions on Applied Superconductivity, Vol. 11, No. 1, March 2001, pp 337-340.
The microstrip line integral to the superconductor chip may be referred to herein as the xe2x80x9con-chipxe2x80x9d microstrip line. The microstrip line formed on the substrate that provides for chip to chip communication may be referred to herein as the xe2x80x9coff-chipxe2x80x9d microstrip line. From the foregoing publications it would appear that using solder bumps to join (and serve as the transition between) the on-chip microstrip to off-chip microstrip line is suggested as an approach to enabling chip to chip communication within a superconductor Multi-Chip Module. Yet, no one reported successfully doing so. The present applicants were also unsuccessful in using that straight forward approach to produce a practicable transition, one that could provide adequate bandwidth. The present applicants found that the solder bump connection that made the transition between the on-chip microstrip and the off-chip microstrip for low characteristic impedance produced an electrical mis-match and a narrow bandwidth.
As is known, to obtain maximum signal power transfer between different transmission lines, the characteristic impedance of the transmission lines must be the same. Ideally, that impedance is frequency independent. As an example, the on-chip and off-chip transmission lines are designed to be four (or eight) ohms in value to match the impedance of the electronic circuits on the chip. The solder bump transition used for signal passage between those lines possesses inductance, capacitance, parasitic resistance, and magnetic coupling to adjacent solder bumps. Those properties of the solder bump are of some significance at the high frequencies (and speeds) involved with superconductor circuits, a characterization that is known from the cited Maezawa publication. Irrespective of the characterization others have made of the solder bump as a transition joining the two microstrip lines, no one appears to have discovered a transition that attains large bandwidth for low characteristic impedance. A need exists to produce a solder bump coupling or transition between on-chip and off-chip microstrip lines that is impedance matched to those lines over a bandwidth of 200 GHz. As an advantage, the present invention achieves that goal.
Accordingly, an object of the invention is to improve signal transmission between separate superconductor chips.
A further object of the invention is to improve the bandwidth of the SFQ pulse transmission path used to propagate SFQ pulses between superconductor chips.
And, a still further object of the invention is to improve the matching between a superconductor integrated circuit chip that incorporates xe2x80x9cflip-chipxe2x80x9d solder bump technology to fasten the chip to the substrate and a microstrip transmission line located on the substrate.
In accordance with the foregoing objects and advantages, a superconductor on-chip microstrip line to off-chip microstrip line transition is realized that obtains bandwidth of 200 GHz for MCM application while employing solder bump technology to connect the superconductor chips to the off-chip microstrip line and substrate. Circular openings through the respective ground plane layers of the off-chip and on-chip microstrip lines are provided in positions respectively underlying and overlying the solder bump for the signal. In accordance with the invention, those openings may be sized to provide desired values of capacitance and inductance, it being found that the larger the size of the openings, the lesser the capacitance value and the greater the inductance. Through design and trial and error, an opening size is achieved to enable the electronic characteristics of the solder bump transition to match the characteristic impedance of the respective microstrip lines over a wide bandwidth.
The foregoing and additional objects and advantages of the invention, together with the structure characteristic thereof, which were only briefly summarized in the foregoing passages, will become more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment of the invention, which follows in this specification, taken together with the illustrations thereof presented in the accompanying drawings.